1. Field of the Invention
The invention relates to aerofoil sections and in particular to those designed to endure turbulent i.e. high Reynolds number air flows. It has particular application to large civil aircraft.
2. Discussion of Prior Art
It is an ever present desire to design aerofoils having improved drag characteristics which at the same time do not diminish lift characteristics to an undesirable level. The parasitic drag on an aircraft includes both viscous drag, resulting from the viscosity of the air, and pressure drag, resulting from an imbalance of pressure acting on the aircraft surfaces. At subsonic velocity the viscous drag is the predominant contributor to the parasitic drag of the aircraft. However, as the aircraft approaches Mach 1.0, local regions of supersonic flow develop on the surface of the aircraft. For local Mach numbers nearly equal to 1.0, the air is able to recompress (return to subsonic conditions) without forming local pressure jump discontinuities or shocks on the surface of the aircraft. U.S. Pat. No. 5,318,249 describes a transonic aerofoil in which the absolute value of the negative slope of the camber increases by at least about 50% over the aft 4% of the chord.
It is an object of the invention to provide an aerofoil with even further improved reduced drag characteristics than prior art aerofoils.
The inventors have determined that applying a pressure distribution on the upper surface of an aerofoil with a mild adverse pressure gradient recovery, followed by a rapid increase in pressure just short of the trailing edge, allows aerofoils to be designed which offer lower drag at high Reynolds numbers. The effect of this is to maximise lift to drag ratio by pushing the boundary layer as hard as possible i.e. close to separation. The inventors have defined a series of upper surface pressure distributions made up of regions of zero pressure gradient, regions of equilibrium, adverse gradient and jumps in pressure. By applying a simplified boundary-layer calculation to the upper surface pressure distributions the inventors have showed that improvements in lift to drag ratios are possible over prior art aerofoils. The inventors have also determined that there was a value of adverse pressure gradient parameter above which the flow would be sensitive to Reynolds number.
In accordance with the present invention an aerofoil wing comprises upper and lower surfaces meeting at the leading and trailing edges and a camber line representing the curvature of the aerofoil characterised in that the camber line comprises at least two regions of substantially linear increases in curvature; wherein in a first region the maximum deviation of linearity in curvature increase from a straight line over at least 20% of chord is 1/chord; wherein in a second region includes a curvature increase of at least 3/chord the maximum deviation of linearity in curvature increase is 1.5/chord from a straight line; and wherein the increases in curvature are of monotonically increasing magnitude.